Pumping method and unit for optical amplifiers

ABSTRACT

Optical pumping unit comprising a first pump source adapted to emit a first pump radiation at wavelength λp 1;  a second pump source adapted to emit a second pump radiation at wavelength λp 2,  with wavelength λp 2  different from wavelength λp 1;  and a common coupling section comprising a first and a second port connected to the first and second pump source for respectively receiving the first and the second pump radiation; a third port for a signal radiation at wavelength λs; a fourth port, wherein the coupling section is adapted to combine, in the fourth port, the signal radiation and the first and second pump radiation through a reversal of the direction of propagation of the first pump radiation from the first port to the fourth port.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S.C. §119 of European Patent Application Ser. No. EP00204828.8 filed on Dec.27, 2000.

[0002] This application claims the benefit of priority under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 60/258,563 filed on Dec.29, 2000.

DESCRIPTION

[0003] The present invention relates to an optical pumping unitcomprising a first and a second pump source for providing two pumpradiations, and a common coupling section for coupling a first, a secondand a third radiation.

[0004] The present invention also relates to an optical amplifiercomprising said optical pumping unit and an optical communication lineand an optical communication system comprising said optical pumping unitor said optical amplifier.

[0005] The present invention also relates to a coupling section and amethod for coupling two pump radiations and a signal radiation.

[0006] In the present description and claims, the expression

[0007] “insertion losses undergone by a pump radiation”, referred to apumping unit, is used to indicate the difference, expressed in dB,between the power of the radiation emitted by a pump source of thepumping unit and the power in output from the pumping unit;

[0008] “100% λ optical coupler” is used to indicate an optical couplercomprising two optical paths coupled to one another and adapted to letpass substantially 100% of power of a radiation of wavelength λ from oneoptical path to the other, and to substantially maintain 0% of poweralong the same optical path;

[0009] “100% λx/0% λy WDM optical coupler” is used to indicate anoptical coupler comprising two optical paths coupled to one another andadapted to let pass from one optical path to the other substantially100% of power of a radiation at wavelength λx, and substantially 0% ofpower of a radiation at wavelength λy by maintaining substantially 0% ofpower of the radiation at wavelength λx and substantially 100% of powerof the radiation at wavelength λy along the same optical path;

[0010] “50% λx/0% λy WDM optical coupler” is used to indicate an opticalcoupler comprising two optical paths coupled to one another and adaptedto let pass from one optical path to the other substantially 50% ofpower of a radiation at wavelength λx and substantially 0% of power of aradiation at wavelength λy by maintaining substantially 50% of power ofthe radiation at wavelength λx and substantially 100% of power of theradiation at wavelength λy along the same optical path;

[0011] “50% λx/100% λy WDM optical coupler” is used to indicate anoptical coupler comprising two optical paths coupled to one another andadapted to let pass from one optical path to the other substantially 50%of power of a radiation at wavelength λx and substantially 100% of powerof a radiation at wavelength λy by maintaining substantially theremaining 50% of power of the radiation at wavelength λx andsubstantially 0% of power of the radiation at wavelength λy along thesame optical path;

[0012] “optical transmission fibre” is used to indicate an optical fibreused in an optical communication line or system for transmitting opticalsignals from a point to another placed at an appreciable distance.

[0013] In the above definitions, the expression “substantially 100%” ofpower coupling is preferably used for indicating a power coupling atleast equal to 90%; “substantially 0%” of power coupling, is preferablyused for indicating a power coupling that is less than or equal to 10%,and “substantially 50%” of power coupling is preferably used forindicating a power coupling comprised between 45% and 55%.

[0014] A 100% λWDM, 100% λx/0% λy WDM, 50% λx/0% λy WDM, 50% λx/100% λyWDM optical coupler can be realised in micro-optics, fused fibre,integrated optics or through any other technique which allows theformation of waveguides at the optical frequencies.

[0015] An optical amplifier typically consists of an active means (forexample, an optical fibre or a planar waveguide doped with a rare earth)and a pumping unit.

[0016] In turn, the pumping unit typically consists of a pump source(for example, a laser diode) adapted to provide a pump radiation atwavelength λp to the active means, and of a wavelength divisionmultiplexing (or WDM) device for coupling the pump radiation atwavelength λp with a signal radiation to be amplified at wavelength λs.

[0017] Typically, the WDM device is a WDM optical coupler of the 100%λp/0% λs or 100% λs/0% λp type, with two inputs and two outputs, and itis adapted to couple—into one of the two outputs—substantially all thepower of the pump radiations at wavelength λp and of the signalradiations at wavelength λs at its two inputs.

[0018] With the advent of WDM optical communication systems, there isthe need of increasing the pump power of optical amplifiers, so as toeffectively amplify a WDM optical signal.

[0019] A WDM optical signal is a signal comprising a plurality of Noptical signals independent of one another, each at a predeterminedcentral wavelength λ1, λ2 . . . λN different from that of the othersignals. The signals can also be both digital and analogue, and theyhave a certain spectral width around the value of the centralwavelength.

[0020] Typically, in a WDM system, the various optical signals aregenerated by a plurality of optical sources, multiplexed so as to form aWDM signal, transmitted along the same optical transmission line (forexample an optical fibre line) and then demodulated so as to be eachreceived by a receiver.

[0021] Optical amplifiers used in transmission, reception and/or along atransmission line of a WDM optical system need high pump powers toefficiently amplify the plurality of optical signals forming the WDMoptical signal.

[0022] For the purpose of meeting said requirement, the use of a pumpingunit with two pump sources has been proposed.

[0023] More in particular, it has been proposed to combine two pumpradiations provided by two pump sources in a single total pumpradiation, and to combine said total pump radiation with the signalradiation.

[0024] For example, the use of a wavelength combiner or of apolarisation combiner—upstream of a WDM device used for coupling thetotal pump radiation with the signal radiation—has been proposed tocombine the two pump radiations.

[0025] In the first case (FIG. 1), the pumping unit 10 comprises twolaser diodes 11, 12, two optical fibre gratings 15, 17 respectivelyconnected to the two lasers 11 and 12, a wavelength combiner 14 and afused fibre 100% λp/0% λs WDM optical coupler 16. According to thissolution, the pump radiations emitted by the two laser diodes 11, 12have slightly different wavelengths (typically, by some nm) and the twooptical fibre gratings 15, 17 are adapted to stabilise said wavelengths.

[0026] Since the wavelengths of the two pump radiations are very closeto one another (by some nm), the wavelength combiner 14 is typicallyrealised in micro-optics. In fact, a fused fibre 100% λp/0% λs WDMoptical coupler of the type used for coupling the total pump radiationand the signal radiation (which typically have different wavelengthsfrom one another, in the range of dozens or hundreds nm), is not adaptedto combine wavelengths that are very close to one another (in the rangeof nm).

[0027] Considering Bragg gratings currently available on the market byJDS, E-TeK, Innovative Fibers, Sumitomo, Bragg Photonics, 3M, OpticalTechnologies Italia (having insertion losses that are more than or equalto about 0.2 dB) and micro-optics wavelength combiners and fused fibre100% λp/0% λs WDM optical couplers currently available on the market byJDS, E-TeK, Oplink and Gould (having insertion losses that arerespectively higher than or equal to about 0.6 dB and 0.3 dB), theApplicant has noted that in the pumping unit of FIG. 1 each pumpradiation undergoes insertion losses higher than about 1.1 dB (that is,higher than about 23%).

[0028] Moreover, as the pumping unit of FIG. 1 consists of themicro-optics wavelength combiner 14, the two optical fibre gratings 15,17 and the fused fibre 100% λp/0% λs WDM optical coupler 16, it isrealised using different technologies. This makes the pumping unit lessreliable and more expensive than a unit that is entirely realised withthe same technology (for example, all in fibre or all in micro-optics).

[0029] In the second case of use of a polarisation combiner (FIG. 2),the pumping unit 10 comprises two laser diodes 11, 12 withpolarisation-holding pigtail 11 a, 12 a, a polarisation combiner 13 anda fused fibre 100% λp/0% λs WDM optical coupler 16. According to thissolution, the pump radiations emitted by the two laser diodes 11, 12have the same wavelength, and the two pigtails 11 a, 12 a make said pumpradiation have orthogonal polarisation states.

[0030] The Applicant checked that in this second pumping unit of FIG. 2,insertion losses undergone by the pump radiations are comparable tothose undergone by the pump radiations in the pumping unit of FIG. 1.

[0031] Moreover, since the pumping unit of FIG. 2 requires the use ofpolarisation-maintaining components, it is difficult and expensive to berealised.

[0032] M. Ohashi et al. (“Novel pump-LD with self wavelength-tuningfunction”, ECOC 2000) describe a pumping module comprising four laserdiodes with emission at the wavelengths of 980, 982, 981, 983 nm, awavelength combiner and a wide band optical fibre Bragg grating (FBG).In turn, the wavelength combiner consists of three cascadedfused-tapered Mach-Zehnder interferometers.

[0033] The pump radiations combined with the described module areintended to be coupled—through a distinct WDM device—with the radiationof the signal to be amplified, to be sent along an active optical fibreof an optical amplifier.

[0034] Nevertheless, the Applicant has noted that also in this case, theinsertion losses undergone by the pump radiations are equally high(about 1.1 dB).

[0035] The Applicant has noted that the above proposed solutions allhave a coupling section of the pump radiation which is clearly distinctfrom the coupling section of the total pump radiation with the signalradiation. This makes it necessary to use a certain number of opticalcomponents in cascade—such as, for example, polarisation combiners, 100%λx/0% λy WDM optical couplers, micro-optics wavelength combiners,Mach-Zehnder interferometers—which introduce undesired insertion losseson the pump radiations.

[0036] Thus, the Applicant faced the technical problem of reducing theinsertion losses undergone by the pump radiations of an optical pumpingunit having at least two pump sources.

[0037] The Applicant has found that, by using a common coupling sectionfor mixing at least two pumping radiations and the signal radiation, theoptical power losses undergone by the pump radiations significantlyreduce.

[0038] Thus, in a first aspect thereof, the present invention relates toan optical pumping unit comprising

[0039] a first pump source adapted to emit a first pump radiation atwavelength λp1;

[0040] a second pump source adapted to emit a second pump radiation atwavelength λp2, with wavelength λp2 different from wavelength λp1; and

[0041] a common coupling section comprising

[0042] a first and a second port connected to the first and second pumpsource for respectively receiving the first and the second pumpradiation;

[0043] a third port for a signal radiation at wavelength λs;

[0044] a fourth port, wherein said coupling section is adapted tocombine, in the fourth port, the signal radiation and the first andsecond pump radiation by means of a reversal of the direction ofpropagation of the first pump radiation from the first port to thefourth port.

[0045] Since the optical pumping unit of the invention uses a commoncoupling section with all of the above features for coupling pump andsignal radiations, it eliminates the need of using distinct couplingsections for combining the pump radiations in a total pump radiation andthe total pump radiation with the signal radiation. Thus, it allowsusing a limited number of optical components in cascade, thus reducingthe insertion losses undergone by the pump radiations.

[0046] Moreover, by using a limited number of optical components incascade, the optical pumping unit of the invention is more compact andless expensive to be realised than the above-mentioned conventionalpumping units.

[0047] In fact, a limited number of optical components allowssimplifying the step of assembly of the pumping unit (thus reducing, forexample, the number of junctions to be made between the components) andlimiting production times and costs.

[0048] Typically, the wavelength λs of the signal radiation is higherthan wavelengths λp1 and λp2 of the pump radiations.

[0049] Advantageously, the difference (λs−λp_(max)) between wavelengthλs and the highest λp_(max) between wavelengths λp1 and λp2 is equal toat least 30 nm. In a preferred embodiment, it is equal to at least 530nm.

[0050] Advantageously, the difference between wavelengths λp1 and λp2 isless than or equal to, 30 nm. Preferably, it is less than or equal to,20 nm. More preferably, it is less than or equal to, 10 nm.

[0051] Typically, in the case of application of the pumping unit forpumping an erbium-doped optical amplifier, wavelengths λp1 and λp2 areselected within an interval of wavelengths comprised between about 975and 985 nm and/or 1470 and 1490 nm whereas wavelength λs is selectedwithin an interval of wavelengths comprised between about 1520 and 1630nm.

[0052] In turn, in the case of application of the pumping unit formixing three signal radiations in the treatment of WDM signals,wavelengths λp1 and λp2 are, for example, selected at about 1530 nm andrespectively, 1550 nm whereas wavelength λs is selected at about 980nm.

[0053] Typically, the coupling section has a first and a second side,the one opposed to the other. Advantageously, the first and the fourthport of the coupling section are located at the first side, while thesecond and the third port are at the second side, in positionsrespectively corresponding to the first and the fourth port.

[0054] Preferably, the coupling section comprises

[0055] a first optical path which connects the first and the secondport; and

[0056] a second optical path, in communication with the first opticalpath, which connects the third and the fourth port, and it is adapted tosend to the fourth port the first pump radiation, which propagates alongthe first optical path from the first port to the second port, making itpass from the first optical path to the second optical path andreflecting it back towards the fourth port.

[0057] Advantageously, the coupling section is also adapted to send tothe fourth port the second pump radiation, which propagates along thefirst optical path from the second port towards the first port, makingit pass from the first optical path to the second optical path.

[0058] Moreover, the coupling section is also preferably adapted to letthe signal radiation propagate along the second optical path.

[0059] According to an embodiment, the signal radiation propagates alongthe second optical path from the third port to the fourth port.

[0060] According to an alternative embodiment, the signal radiationpropagates along the second optical path from the fourth port towardsthe third port.

[0061] Advantageously, the coupling section comprises an opticalreflection element adapted to reflect the first pump radiation atwavelength λp1 towards the fourth port, and to let the second pumpradiation at wavelength λp2 and the signal radiation at wavelength λspass.

[0062] Preferably, said optical reflection element is a Bragg grating.As an alternative, said optical reflection element is a thin-filmoptical filter, such as a Fabry-Perot interferometer.

[0063] Preferably, the coupling section is of the interferometric type.

[0064] Advantageously, the first optical path comprises a waveguide.

[0065] Advantageously, the second optical path comprises a waveguide.

[0066] Preferably, said waveguide is an optical fibre. According to analternative, it is a planar waveguide realised in integrated optics.

[0067] Preferably, the first and the second optical path are coupledalong a coupling area.

[0068] More preferably, the coupling area is such as to letsubstantially all the power of the signal radiation at wavelength λspropagate along the second optical path, and to let substantially allthe power of the first pump radiation at wavelength λp1 andsubstantially all the power of the second pump radiation at wavelengthλp2 pass from the first optical path to the second optical path.

[0069] Advantageously, the first and the second optical path form a WDMoptical coupler of the 100% λp1, λp2/0% λs type, comprising twowaveguides coupled with one another in said coupling area.

[0070] Preferably, the 100% λp1, λp2/0% λs WDM optical coupler is afused fibre optical coupler. According to an alternative, it is realisedin integrated optics (for example, in planar waveguide).

[0071] Preferably, the optical reflection element is positioned in thecoupling area of the first and the second optical path.

[0072] More preferably, the optical reflection element is positioned ina point of the coupling area at which about 50% of power of the firstpump radiation passes from the first optical path to the second opticalpath.

[0073] According to an embodiment, the first and the second optical pathare also coupled along a second coupling area.

[0074] In the optical pumping unit according to this embodiment, thefirst and the second optical path advantageously comprise an inputcoupler, an output coupler, an upper arm and a lower arm. Moreover, theinput coupler has four ports of which two are the second and the thirdport of the coupling section, and two are in communication with theupper arm and the lower arm, while the output coupler has four ports ofwhich two are the first and the fourth port of the coupling section, andtwo are in communication with the upper arm and the lower arm.

[0075] In the optical pumping unit according to this embodiment, thecoupling section preferably comprises also a second optical reflectionelement adapted to reflect the first pump radiation at wavelength λp1towards the fourth port, and to let the second pump radiation atwavelength λp2 and the signal radiation at wavelength λs pass, the firstoptical reflection element being arranged in said upper arm and thesecond optical reflection element being arranged in said lower arm.

[0076] The input coupler and the output coupler preferably are two WDMoptical couplers of the 50% λp1, λp2/0% λs type, each comprising twowaveguides coupled with one another in said first and said secondcoupling area.

[0077] The input and output optical couplers and the two upper and lowerarms are preferably realised in optical fibre. According to analternative, they are realised in integrated optics (for example, inplanar waveguide).

[0078] In a second aspect thereof, the invention also relates to anoptical amplifier for amplifying a signal radiation at wavelength λscomprising a dielectric guiding active means and a pumping unit of thetype described above with reference to the first aspect of the inventionwherein the fourth port of the coupling section is in communication withthe active means.

[0079] Advantageously, the active means is an optical waveguide dopedwith at least one rare earth. Typically, said at least one rare earth iserbium.

[0080] Typically, the doped optical waveguide is an optical fibre or aplanar waveguide realised in integrated optics.

[0081] In a third aspect thereof, the invention also relates to anoptical communication line comprising a transmission optical fibrelength and a pumping unit of the type described above with reference tothe first aspect of the invention wherein the fourth port of thecoupling section is in communication with said transmission opticalfibre length.

[0082] In a fourth aspect thereof, the invention also relates to anoptical communication line comprising a transmission optical fibrelength and an optical amplifier, of the type described above withreference to the second aspect of the invention, in communication withsaid transmission optical fibre length.

[0083] In a fifth aspect thereof, the present invention also relates toan optical communication system comprising

[0084] a transmitting station adapted to provide a signal radiationhaving wavelength λs;

[0085] an optical transmission line, optically connected to saidtransmitting station, for transmitting said signal radiation;

[0086] a receiving station, optically connected to said opticaltransmission line, for receiving said signal radiation;

[0087] at least one pumping unit, of the type described above withreference to the first aspect of the invention, in communication withsaid optical transmission line.

[0088] In a sixth aspect thereof, the present invention also relates toan optical communication system comprising

[0089] a transmitting station adapted to provide a signal radiationhaving wavelength λs;

[0090] an optical transmission line, optically connected to saidtransmitting station, for transmitting said signal radiation;

[0091] a receiving station, optically connected to said opticaltransmission line, for receiving said signal radiation;

[0092] at least one optical amplifier, of the type described above withreference to the second aspect of the invention, in communication withsaid optical transmission line.

[0093] Advantageously, said transmitting station is adapted to provide aWDM optical signal comprising a plurality of N signals havingwavelengths λ1, λ2 . . . λN.

[0094] In this case, said receiving station is advantageously adapted toreceive and demultiplex said WDM optical signal.

[0095] In a seventh aspect thereof, the present invention also relatesto an optical coupling section for coupling a signal radiation atwavelength λs, a first pump radiation at wavelength λp1 and a secondpump radiation at wavelength λp2, comprising

[0096] a first and a second port for receiving respectively the firstand the second pump radiation;

[0097] a third port for the signal radiation; and

[0098] a fourth port, and being adapted to combine the signal radiationand the first and second pump radiation in the fourth port through areversal of the direction of propagation of the first pump radiationfrom the first port to the fourth port.

[0099] As regards the features of the coupling section and of the pumpand signal radiations, reference shall be made to what described abovewith reference to the pumping unit according to the first aspect of theinvention.

[0100] In an eighth aspect thereof, the present invention also relatesto an optical coupling section, for coupling a first radiation atwavelength p1, a second radiation at wavelength λp2 and a thirdradiation at wavelength λs, comprising

[0101] a first and a second port for respectively receiving the firstand the second radiation;

[0102] a third port for the third radiation; and

[0103] a fourth port, and being adapted to combine the first, the secondand the third radiation in the fourth port through a reversal of thedirection of propagation of the first radiation from the first port tothe fourth port.

[0104] As regards the features of the coupling section of the first,second and third radiation, reference shall be made to what describedabove with reference to the pumping unit according to the first aspectof the invention, and with reference to the first and second pumpradiation and to the signal radiation.

[0105] In a further aspect thereof, the present invention also relatesto a method for coupling a first radiation at wavelength λp1, a secondradiation at wavelength λp2 and a third radiation at wavelength λsthrough a common coupling section having a first and a second side thatare opposed to one another, the first side comprising a first and afourth port and the second side comprising a second and a third port,said method comprising the steps of

[0106] a) propagating the second radiation from the second port to thefirst port;

[0107] b) deviating the path of the second radiation so as to send it tothe fourth port;

[0108] c) sending the third signal radiation from the third port to thefourth port, or vice versa, from the fourth port to the third port;

[0109] d) propagating the first radiation from the first port to thesecond port; and

[0110] e) reversing the direction of propagation of the first radiationto send it to the fourth port.

[0111] Advantageously, the common coupling section also comprises afirst optical path connecting the first and the second port, and asecond optical path, in communication with the first optical path,connecting the third and the fourth port.

[0112] Preferably, step a) is carried out by sending the secondradiation along the first optical path from the second port to the firstport.

[0113] Moreover, step b) is preferably carried out by making the secondradiation pass from the first optical path to the second optical path.

[0114] Advantageously, step c) is carried out by letting the thirdradiation propagate along the second optical path.

[0115] Preferably, step d) is carried out by sending the first radiationalong the first optical path from the first port to the second port.

[0116] Moreover, step e) is preferably carried out by making the firstradiation pass from the first optical path to the second optical pathand back-reflecting it towards the fourth port.

[0117] Advantageously, the passage from the first to the second opticalpath of steps b) and e) occurs by interferometric effect.

[0118] As regards the features of the coupling section and of the first,second and third radiation, reference shall be made to what describedabove with reference to the pumping unit according to the first aspectof the invention and to the first and second pump radiation and to thesignal radiation.

[0119] Features and advantages of the invention shall now be describedwith reference to embodiments shown by way of a non-limiting example inthe attached drawings. In such drawings:

[0120]FIG. 1 describes a first embodiment of a pumping unit according tothe prior art;

[0121]FIG. 2 describes a second embodiment of a pumping unit accordingto the prior art;

[0122]FIGS. 3a, 3 b and 3 c show three embodiments of an optical pumpingunit according to the invention;

[0123]FIG. 4 shows a first embodiment of a coupling section according tothe invention;

[0124]FIG. 5 shows a second embodiment of a coupling section accordingto the invention;

[0125]FIG. 6 shows an embodiment of a pumping unit according to theinvention, having the coupling section of FIG. 4;

[0126]FIG. 7 shows an alternative embodiment of the pumping unit of FIG.6;

[0127]FIG. 8 shows an embodiment of a pumping unit according to theinvention, having the coupling section of FIG. 5;

[0128]FIGS. 9a and 9 b show two alternative embodiments of a pumpingunit according to the invention with three pump sources;

[0129]FIG. 10 shows an equipment used for producing a fused fibre WDMoptical coupler of the 100% λp/0λs type;

[0130]FIG. 11 shows the pattern of the optical power detected by twodetectors of the equipment of FIG. 10 at wavelengths λs and λp infunction of the elongation (L) of the fused fibres and the ratio betweenthe two detected optical powers;

[0131]FIG. 12 shows a writing step of a Bragg grating on a fused fibreWDM coupler of the 100% λp/0% λs type;

[0132]FIG. 13 shows a trimming step subsequent to the writing step ofFIG. 12;

[0133]FIGS. 14a and 14 b show two alternative embodiments of an opticalamplifier according to the invention;

[0134]FIG. 15 shows an embodiment of an optical communication systemaccording to the invention.

[0135]FIG. 3a shows an optical pumping unit 10 according to theinvention, comprising a first pump source 11, a second pump source 12and a common interferometric coupling section 20.

[0136] The first and the second pump source 11 and 12 are, for example,two laser diodes adapted to provide pump radiations having wavelengthsλp1 and λp2 that are selected according to the applications of thepumping unit 10.

[0137] The coupling section 20 comprises a first 21 and a second 22 portfor respectively receiving the first pump radiation and the second pumpradiation; an input/output third port 23 for a signal radiation atwavelength λs (or for a WDM signal comprising a plurality of signalradiations at wavelengths λ1, λ2 . . . λN) and an input/output fourthport 24 for said pump and signal radiations.

[0138] The first and the fourth port 21, 24 are on an opposed side ofthe coupling section 20 with respect to that of the second and thirdport 22, 23. Moreover, the first port 21 is in a position correspondingto the second port 22, whereas the fourth port 24 is in a positioncorresponding to the third port 23.

[0139] As shown in FIG. 3a, the coupling section is adapted to combinethe first and the second pump radiation and the signal radiation intoport 24 through a reversal of the direction of propagation of the firstpump radiation from the first port 21 towards the fourth port 24.

[0140] In the embodiment of FIG. 3b, the first 21 and the second 22 portare connected through a first optical path 25 whereas the third andfourth port are connected through a second optical path 26 incommunication with the first optical path 25.

[0141] Moreover, the coupling section 20 is adapted to send, to thefourth port 24

[0142] the first pump radiation, which propagates along the firstoptical path 25 from the first port 21 towards the second port 22,making it pass from the first optical path 25 to the second optical 26and reversing its direction of propagation;

[0143] the signal radiation, which propagates along the second opticalpath 26 from the third port 23 to the fourth port 24, letting itpropagate along the second optical path 26;

[0144] the second pump radiation, which propagates along the firstoptical path 25 from the second port 22 towards the first port 21,making it pass from the first optical path 25 to the second optical path26.

[0145] Thus, at the fourth port 24, the signal and pump radiations areco-propagating.

[0146]FIG. 3c shows an alternative embodiment of the optical pumpingunit 10 which is totally similar to that of FIG. 3b except in that, atport 4, the pump radiations propagate in a counter-propagating directionwith respect to the direction of the signal radiation.

[0147] In fact, in the embodiment of FIG. 3c, the fourth port 24functions as output port for the first and the second pump radiation,and as input port for the signal radiation. Moreover, the third port 23functions as output port for the signal radiation.

[0148] In addition, the coupling section 20 is adapted to send thesignal radiation to the third port 23, which propagates along the secondoptical path 26 from the fourth port 24 to the third port 23, letting itpropagate along the second optical path 26.

[0149]FIG. 4 shows a first embodiment of a common interferometriccoupling section 20 according to the invention.

[0150] In such embodiment, the first and the second optical path 25 and26 comprise two optical fibres forming a 100% λp1, λp2/0% λs WDM fusedfibre optical coupler. More in particular, the 100% λp1, λp2/0% λs WDMoptical coupler consists of two fibres, fused with one another in acoupling area 28 such as to allow a coupling from one fibre to the otherof substantially 100% of power of the pump radiations at wavelengths λp1and λp2 and of substantially 0% of power of the signal radiation atwavelength λs (or, in the case of WDM signal, at wavelengths λ1, λ2 . .. λN).

[0151] In this way, in the coupling section 20,

[0152] the signal radiation is let to propagate from the third port 23to the fourth port 24 along the second optical path 26; whereas

[0153] the second pump radiation is let to propagate, along the firstoptical path 25, from the second port 22 towards the first port 21 up tothe coupling area 28, where substantially 100% of its power is coupledto the optical fibre of the second optical path 26, where it propagatesup to the fourth port 24.

[0154] Moreover, the coupling section 20 comprises an optical reflectionelement 27 having a reflection spectrum comprised in a band ofwavelength Δλp1 centred at about λp1 such as to reflect the first pumpradiation at wavelength Δp1 and to let the second pump radiation atwavelength λp2 and the signal radiation(s) at wavelength(s) λs/λ1, λ2 .. . λN pass.

[0155] Advantageously, in the embodiment shown, the optical reflectionelement 27 is an optical fibre Bragg grating.

[0156] Moreover, it is preferably written in the coupling area 28 of the100% λp1, λp2/0% λs WDM optical coupler at the point in which a transferof substantially 50% of power of the first pump radiation has beenreached between one fibre to the other of the coupler.

[0157] In this way, in the coupling section 20, the first pump radiationis let to propagate, along the first optical path 25, from the firstport 21 towards the second port 22 up to the coupling area 28. In thecoupling area 28, the first pump radiation is then back-reflectedtowards the fourth port 24 by the optical reflection element 27 andsubstantially 100% of its optical power is coupled to the optical fibreof the second optical path 26.

[0158] In fact, the 100% p1, λp2/0% λs WDM optical coupler with theBragg grating 27 written in the coupling area 28 behaves towards thefirst pump radiation at wavelength λp1, like an optical 100% λp1 Braggreflecting coupler (or BRC) as described, for example, by Raman Kashyap,“Fiber Bragg Gratings”, Academic Press, pages 276-284.

[0159] However, in the mentioned reference, the Bragg grating is writtenon a coupler of the 100% λ type, whereas in the coupling section 20 itis written on a 100% λp1, λp2/0% λs WDM coupler. As regards to this, itis worth noting that the function performed by the 100% λp1, λp2/0% λsWDM coupler of combining a pump radiation λp with a signal radiation λscannot be carried out by a coupler of the 100% λ type—which is, forexample, used for realising an optical 100% λBragg reflecting couplerfor adding/dropping channels of a WDM system—since said coupler is notwavelength selective.

[0160] Thus, the coupling section 20 of FIG. 4 allows coupling the firstpump radiation, the second pump radiation and the signal radiation inthe fourth port 24 by using only a 100% λp1, λp2/0% λs WDM opticalcoupler with a Bragg grating 27 suitably written in the coupling area28.

[0161] With respect to the prior art, wherein the first and the secondpump radiation and the signal radiation are coupled by using twodistinct devices in cascade (for example, a wavelength combiner 14 and a100% λp1, λp2/0% λs WDM optical coupler 16 or a polarisation combiner 13and a 100% λp1, λp2/0% λs WDM optical coupler 16, as shown in FIGS. 1and 2), the coupling section 20 according to the invention uses alimited number of passive optical components, and thus it allowsreducing insertion losses undergone by pump radiations.

[0162] In fact, the Applicant has ascertained that the 100% λp1, λp2/0%λs WDM optical coupler with a Bragg grating 27 written in the couplingarea 28 introduces insertion losses lower than about 0.4 dB.

[0163] Such losses are much lower than the insertion losses of 0.9 dBintroduced, as already said above, by the cascade of the wavelengthcombiner 14 and of the 100% λp1, λp2/0% λs WDM optical coupler 16 or bythe cascade of the polarisation combiner 13 and of the 100% λp1, λp2/0%λs WDM optical coupler 16 of FIGS. 1 and 2, considering the devicescurrently available on the market by JDS, E-TeK, Oplink and Gould.

[0164] Moreover, since the coupling section 20 of FIG. 4 has a limitednumber of passive optical components, it is more compact and reliablethan those of the prior art.

[0165]FIG. 5 shows a second embodiment of the coupling section 20according to the invention.

[0166] In such embodiment, the first and the second optical path 25 and26 comprise an input optical coupler 36, an upper arm 38, a lower arm 39and an output coupler 37. The input coupler 36 has a first and a secondport respectively corresponding to the second port 22 and to the thirdport 23 of the coupling section 20 and a third and a fourth portrespectively connected to two ends of the upper arm 38 and of the lowerarm 39. In turn, the output coupler 37 has four ports, of which two areconnected to two ends of the upper arm 38 and of the lower arm 39 andthe other two correspond to the first 21 and to the fourth 24 port ofthe coupling section 20. Moreover, the upper arm and the lower arm 38and 39 respectively house a first and a second optical reflectionelement 27, 27′.

[0167] Couplers 36 and 37 are WDM optical couplers of the 50% λp1,λp2/0% λs type.

[0168] Moreover, the two optical reflection elements 27, 27′ are twosubstantially identical Bragg gratings having a reflection spectrumcomprised in a band of wavelength Δp1 centred at about λp1 such as toreflect the first pump radiation at wavelength λp1 and to let the secondpump radiation at wavelength λp2 and the signal radiation(s) atwavelength(s) λs/λ1, λ2 . . . λN pass.

[0169] In addition, the optical paths covered by the two pump radiationsat wavelengths λp1 and λp2 through the two upper and lower arms 38, 39are preferably balanced.

[0170] Advantageously, the coupling section 20 of FIG. 5 is realised inall-fibre technology, and the two Bragg gratings 27, 27′ are written onthe two upper and lower arms 38, 39 in optical fibre.

[0171] In this case, the first and the second optical path 25 and 26preferably consist of only two portions of optical fibre suitablycoupled to one another in a first 28 and a second 28′ fusion couplingarea so as to obtain the two optical couplers 36 and 37 and the two arms38 and 39 of the coupling section 20 of FIG. 5.

[0172] Such configuration of the coupling section 20, realised inall-fibre technology and consisting of only two portions of opticalfibre suitably coupled with one another in two fusion areas 28, 28′, isadvantageous because, since it does not exhibit internal junctionsbetween the input and output couplers 36 and 37, the upper and lowerarms 38 and 39 and the reflection elements 27, 27′, it allows reducinginsertion losses.

[0173] When the second pump radiation enters in the coupling section 20of FIG. 5 through the second port 22, its optical power is split intotwo substantially equal components by the input optical coupler 36(which is of the 50% λp1, λp2/0% λs type). Such components pass throughthe two upper 38 and lower 39 arms of the modified interferometerpassing through the optical reflection elements 27, 27′ and theycontinue towards the output coupler 37, where they recombine and exitfrom the coupling section 20 through the fourth port 24. In fact, whenthe second pump radiation enters in the coupling section 20 through thesecond port 22, its two components that propagate in the two upper 38and lower 39 arms interfere constructively in the fourth port 24 anddestructively in the first port 21. This is due to the fact that, asknown, an optical signal passing through a 3 dB optical coupler (of the50% λp1, λp2 type) by passing from a waveguide of the coupler to theother, undergoes a 90° phase shift with respect to the optical signalwhich passes through it by remaining in the same waveguide.

[0174] Thus, when the second pump radiation enters through the secondport 22 of the coupling section, it exits—for an interferometriceffect—from the fourth port 24.

[0175] In turn, when the signal radiation enters in the coupling section20 of FIG. 5 through the third port 23, its optical power completelypasses in the lower arm 39 through the input optical coupler 36 (whichis of the 50% λp1, λp2/0% λs WDM type). Then, the signal radiationcontinues along the lower arm 39, where it passes through the opticalreflection element 27′ until it arrives into the output coupler 37which, as it is of the 50% λp1, λp2/0% λs type, lets it exit from thefourth port 24 of the coupling section 20.

[0176] Finally, when the first pump radiation enters in the couplingsection 20 of FIG. 5 through the first port 21, its optical power issplit into two substantially equal components by the output opticalcoupler 37 (which is of the 50% λp1, λp2/0% λs type). Then, saidcomponents pass in the two upper 38 and lower 39 arms of theinterferometer, they are back-reflected by the two optical reflectionelements 27, 27′, return towards the output coupler 37, where theyrecombine and exit from the coupling section 20 through the fourth port24. In fact, for the reason already described above, the two componentsof the first pump radiation interfere constructively in the fourth port24 and destructively in the first port 21.

[0177] It is worth noting that the same effect can be equally obtainedby using two optical couplers 36, 37 of the 50% λp1, λp2/100% λs WDMtype in place of the two couplers of the 50% λp1, λp2/0% λs WDM type,with the only difference that the signal radiation travels on the upperarm 38 rather than on the lower arm 39.

[0178]FIG. 6 shows an embodiment of the pumping unit 10 comprising acoupling section of the type described above with reference to FIG. 4,two pump sources 11, 12 and two stabilisation elements 15, 17 adapted tostabilise the emission wavelength of the two pump sources 11, 12 aroundthe desired value of λp1 and λp2.

[0179] The two pump sources 11, 12 are, for example, two laser diodeswith emission at about 980 and 984 nm having two pigtails.

[0180] Thus, the WDM optical coupler is of the 100% 980, 984 nm/0% λstype (with λs equal, for example, to about 1550 nm).

[0181] Moreover, the two stabilisation elements 15, 17 preferably aretwo Bragg gratings written on the two pigtails of the pump sources 11,12 having reflection spectrums Δλ1 and Δλ2 respectively centred at aboutλp1 (in the example equal to 984 nm) and λp2 (in the example equal to980 nm) and such as to ensure distinct spectrum intervals between thetwo pump sources 11, 12. In the example shown, the band of thereflection spectrums Δλ1 and Δλ2 is preferably less than 8 nm. Forexample, the band is of 2 nm.

[0182] The applicant has noted that the pumping unit of FIG. 6introduces insertion losses on the pump radiations that are less than orequal to, about 0.6 dB (of which 0.2 dB are due to the stabilisationBragg gratings 15, 17 and 0.4 dB are due to the coupling section 20).

[0183] Thus, the pumping unit 10 introduces lower insertion losses onthe pump radiation with respect to those (higher by about 1.1 dB)introduced by the pumping units of the prior art described above.

[0184]FIG. 7 shows an embodiment of the pumping unit 10 totally equal tothat of FIG. 6 except in that the stabilisation element 15 at the outputof the first pump source 11 is not present. Moreover, the couplingsection 20 is realised so that the reflection element 27 reflects aminimum percentage (for example, about 4%) of the first pump radiationtowards the first pump source 11.

[0185] The desired percentage value of power of the first pump radiationback-reflected towards the first pump source 11 can be obtained throughsuitable U.V trimming operations, described hereinafter with referenceto FIG. 13, on the 100% λp1, λp2/0% λs WDM coupler.

[0186] An alternative method to the UV trimming consists in writing theBragg grating in the coupling area 28 of the 100% λp1, λp2/0% λs couplerat a point in which the coupling is slightly (by about 2%) less than50%. In this way, considering the forward and backward path of the firstpump radiation, about 4% of reflected power of the first pump radiationon the first port 21 is obtained.

[0187] This embodiment of FIG. 7 is preferred with respect to that ofFIG. 6 because, since the reflection element 27 is adapted to reflect apercentage of the first pump radiation towards the first pump source 11,it allows eliminating the presence of the stabilisation element 15, thusreducing the number of passive optical components of the pumping unit10.

[0188]FIG. 8 shows an embodiment of the pumping unit 10 totally similarto that of FIG. 6 except in that the coupling section 20 is of the typedescribed above with reference to FIG. 5.

[0189] Similarly to the pumping unit of FIG. 7, also in the case of FIG.8 the stabilisation element 15 can be omitted and the reflectionelements 27, 27′ can be suitably positioned on the upper and lower arms38, 39 so as to reflect a suitable percentage of power of the first pumpradiation towards the first pump source 11.

[0190] As an alternative, the same effect can be obtained with suitableU.V. trimming operations.

[0191]FIG. 9a shows an embodiment of a pumping unit 10 which allowscombining a first, a second and a third pump radiation at wavelengthsλp1, λp2 and λp3 with the signal radiation(s) at wavelength(s) λs/λ1, λ2. . . λN.

[0192] Such pumping unit 10 comprises three pump sources 11, 12 and 18,three stabilisation elements 15, 17, 19 and a coupling section 20comprising two coupling subsections 20′, 20″.

[0193] As regards the features of the three pump sources 11, 12 and 18and of the three stabilisation elements 15, 17, 19 reference shall bemade to what described above with reference to FIGS. 3-8.

[0194] As regards the coupling subsection 20′, it is totally similar toone of the coupling sections described above with reference to FIGS. 4and 5, except in that the WDM optical couplers are of the 100% p1, λp2,λp3/0% λs type (in case of a configuration similar to that of FIG. 4) orof the 50% λp1, λp2, λp3/0% λs type (in case of a configuration similarto that of FIG. 5).

[0195] Moreover, as regards the coupling subsection 20″, it is totallyequal to one of the coupling sections described with reference to FIGS.4 and 5, except in that the reflection element(s) 27, 27′ have areflection spectrum centred at about λp3 and the WDM optical couplersare of the 100% λp2, λp3/0% λs type (in case of a configuration similarto that of FIG. 4) or of the 50% λp2, λp3/0% λs type (in case of aconfiguration similar to that of FIG. 5).

[0196] Moreover, as regards the coupling subsection 20″, it is worthnoting that, since it does not have to couple the signal radiation, itdoes not need WDM optical couplers of the 100% λp2, λp3/0% λs or 50%λp2, λp3/0% λs type, and can thus use conventional optical couplers ofthe 100% λp2, λp3 or 50% λp2, λp3 type (that is, of the 3 dB type).

[0197]FIG. 9b shows an alternative embodiment of the pumping unit 10 ofFIG. 9a for combining a first, a second and a third pump radiation atwavelengths λp1, λp2 and λp3 with the signal radiation(s) atwavelength(s) λs/λ1, λ2 . . . λN.

[0198] In the embodiment of FIG. 9b, the coupling subsection 20′ istotally equal to one of the coupling sections described with referenceto FIGS. 4 and 5, except in that the WDM optical couplers are of the100%, p1, λp2, λp3/0% λs type (in case of a configuration similar tothat of FIG. 4) or of the 50% λp1, λp2, λp3/0% λs type (in case of aconfiguration similar to that of FIG. 5) and the reflection element(s)27, 27′ are adapted to reflect both wavelengths λp1 and λp2.

[0199] Moreover, the coupling subsection 20″ is totally equal to one ofthe coupling sections described with reference to FIGS. 4 and 5.

[0200] However, similarly to what described in relation to FIG. 9a,since the coupling subsection 20″ does not have to couple the signalradiation, it does not need WDM optical couplers of the 100% λp1, λp2/0%λs type or of the 50% λp1, λp2/0% λs type, and can thus use conventionaloptical couplers of the 100% λp1, λp2 or 50% λp1, λp2 type (that is, ofthe 3 dB type).

[0201] Also according to the two embodiments of FIGS. 9a and 9 b withthree pump sources, the pumping unit 10 of the invention uses a limitednumber of optical components. Thus, it allows reducing the insertionlosses introduced on the pump radiations with respect to the pumpingunits of the prior art described above which, by first combining thepump radiations in a total pump radiation, and then the total pumpradiation with the signal radiation by means of optical devices that arewell distinct from one another, use a greater number of opticalcomponents and have higher insertion losses.

[0202] Should it be necessary to combine M pump sources with the signalradiation(s), the pumping unit 10 will comprise M pump sources and M-1coupling subsections in a way similar to what described with referenceto FIGS. 9a and 9 b.

[0203]FIG. 10 shows an example of an equipment 40 used by the Applicantfor producing a fused fibre WDM coupler of the 100λp/0% λs type,comprising a micro-furnace 41 in which the fibres of a coupler are fusedin the coupling area 28; a pair of motors 42 and 42′ for carrying outthe elongation of the fibres at both sides; two sources 43 and 43′(which respectively emit a radiation at wavelength λs—for example, equalto about 1550 nm—and a radiation at wavelength λp—for example, centredat about 980 nm) and two optical signal detectors 44 and 44′respectively calibrated on the two wavelengths λs and λp. Moreover,equipment 40 comprises two conventional WDM couplers (for example, byE-Tek) 48 and 48′ for mixing and splitting the two radiationsrespectively at wavelengths λs and λp; a radiofrequency generator 45 forheating, by induction, through a spiral winding 49, the micro-furnace41; a pyrometer 46 for measuring the temperature outside micro-furnace41 and a calculator 47 for controlling the fusion process.

[0204] Micro-furnace 41 consists of a platinum hollow cylinder having alength of about 13 mm.

[0205] At the moment of producing the 100% λp/0% λs WDM optical fibrecoupler, the external coating of two optical flbres is removed by alength of about 35 mm, and the two fibres are fastened to motors 42 and42′.

[0206] At this point, the temperature of micro-furnace 41 is brought toabout 1580° C. and a pre-fusion of the optical fibres is carried out inthe coupling area 28 for about 30 sec. Afterwards, a step is started forelongating the fibres by making motors 42, 42′ move at the speed ofabout 45 mm/sec. During this step, the power of radiations atwavelengths λs and λp is constantly kept under control respectively bythe two detectors 44 and 44′.

[0207]FIG. 11 shows the pattern of the optical power detected by the twodetectors 44 and 44′ at the two wavelengths λs and λp (respectively withcurves D and B) in function of the elongation (L) to which the twofibres are subject during the fusion-elongation process, and the ratiobetween the optical power detected at the pump wavelength λp and theoptical power detected at the signal wavelength λs (curve C).

[0208] The fusion and elongation process is stopped when the ratiobetween the optical power at the pump wavelength λp and the opticalpower at the signal wavelength λs reaches a value equal to about 1/1000(point A of FIG. 11).

[0209] The optical features of a 100% 980 nm/0% 1550 nm WDM opticalcoupler obtained by the Applicant with the above method and using twoinitial optical fibres of the Flexcor 1060 model, produced by Corning,are shown in table 1. TABLE 1 IL (port 22 → port 24) @ 980 nm ˜0.35 dBIL (port 23 → port 24) @ 1550 nm ˜0.30 dB Crosstalk (port 22 → port 21)@ 980 nm ˜26 dB Crosstalk (port 23 → port 21) @ 1550 nm ˜26 dB

[0210] where

[0211] the expression “IL (port 22 → port 24) @ 980 nm” indicates theinsertion losses undergone by the radiation at 980 nm passing throughthe WDM coupler from the second port 22 to the fourth port 24 (asregards the numbering of the WDM coupler ports, reference shall be madeto FIG. 4), that is, it indicates the difference, expressed in dB,between the optical power of the radiation at 980 nm in input to thesecond port 22 and that in output from the fourth port 24;

[0212] the expression “IL (port 23 → port 24) @ 1550 nm” indicatesinsertion losses undergone by the radiation at 1550 nm passing tothrough the WDM coupler from the third port 23 to the fourth port 24;

[0213] the expression “Crosstalk (port 22 → port 21) @ 980 nm” indicatesthe difference, expressed in dB, between the optical power of theradiation at 980 nm in input to the second port 22 and that in outputfrom the first port 21;

[0214] the expression “Crosstalk (port 23 → port 21) @ 1550 nm”indicates the difference, expressed in dB, between the optical power ofthe radiation at 1550 nm in input to the third port 23 and that inoutput from the first port 21.

[0215]FIGS. 12 and 13 schematically show two successive steps of a writeoperation of a Bragg grating 27 in the coupling area 28 of a fused fibreWDM coupler of the 100% λp/0% λs type.

[0216] More in particular, FIG. 12 shows a first writing step of grating27 through the use of a phase mask 51, a U.V. radiation source 52, alight source 53 connected to the first port 21 of the WDM coupler, andan optical spectrum analyser 54 connected to the fourth port 24.

[0217] The light source 53 emits a wideband radiation containing thewavelength λp1 of the first pump radiation (for example equal to 984nm).

[0218] The phase mask 51 and the U.V. radiation source 52 are suitablyarranged so as to realise the Bragg grating 27 in the point where theWDM coupler couples from one fibre to the other substantially 50% ofoptical power of the radiation at wavelength λp1.

[0219] Moreover, the U.V. radiation source 52 is kept on until theoptical spectrum analyser 54 detects the desired optical power atwavelength λp1.

[0220]FIG. 13 shows a trimming step of the optical path length of thecoupling area 28 which allows correcting possible errors of positioningof grating 27 with respect to the desired position.

[0221] Said trimming step is carried out through a U.V. radiation source52, a laser diode 56 stabilised at the emission wavelength λp1 through,for example, a Bragg grating (not is shown) on its output pigtail, andthree power meters 57, 58, 59 respectively connected to the first 21,the fourth 24 and the third 23 port of the WDM coupler.

[0222] The laser diode 56 and the power meter 57 are connected to thefirst port 21 through a conventional three-port optical circulator 60.

[0223] The UV trimming step consists in illuminating a limited portionof the coupling area 28 with the help of an optical focusing lens 55 soas to locally vary its mean refractive index and cause a variation ofthe optical path of the coupling area 28.

[0224] The illumination is continued until the following insertion loss(IL) values are reached:

[0225] IL (port 21 → port 24) @ λp1 ≦0.5 dB

[0226] IL (port 21 → port 23) @ λp1 ≧20 dB

[0227] IL (port 21 → port 21) @ λp1 ˜14 dB

[0228] The insertion loss value of 14 dB from port 21 to port 21 allowsobtaining the necessary back-reflected power value of the radiationhaving wavelength Api for stabilising the laser diode 11 when it isjointed to port 21 of the coupling section 20 without the stabilisationelement 15 (see pumping unit of FIG. 7).

[0229] Starting from the above mentioned insertion loss values measuredfor the WDM coupler with grating 27 written in the coupling area 28, theinsertion loss (IL) values of the different paths of said WDM couplerhave been calculated in the case of wavelength λs equal to 1550 nm,wavelength λp1 equal to 984 nm and wavelength λp2 equal to 980 nm. Theresults are summarised in Table 2: TABLE 2 λ INPUT OUTPUT IL 1550 nmPort 23 Port 24 ≦0.35 dB 1550 nm Port 23 Port 21 ≧25 dB  980 nm Port 22Port 24 ≦0.40 dB  980 nm Port 22 Port 23 ≧24 dB  980 nm Port 22 Port 22≧25 dB  980 nm Port 22 Port 21 ≧26 dB  984 nm Port 21 Port 22 ≧31 dB 984 nm Port 21 Port 23 ≧26 dB  984 nm Port 21 Port 21 ˜14 dB  984 nmPort 21 Port 24 ≦0.5 dB

[0230] As regards the method for producing the WDM coupler with grating27 written in the coupling area 28, it is worth noting that the writingof grating 27 causes an increase of the mean refractive index and thus,of the optical path length of the coupling area 28.

[0231] Since the optical path length of the coupling area 28 determines,as shown in FIG. 11, the quantity of coupled optical power from onefibre to the other of the 100% λp/0% λs WDM optical coupler, the writingof grating 27 can change the performances of such coupler.

[0232] For this reason, during the production-of the WDM coupler, theabove fusing and elongation step is stopped when the ratio between thepowers at the two different wavelengths λp and λs reaches a slightlyhigher value than 1/1000 (a little before point A of FIG. 11). Then,after the writing step of grating 27 in the coupling area 28, a U.V.trimming step is carried out, adapted to adjust the optical path lengthof the coupling area 28 so as to obtain a value of the ratio between thepowers at the two wavelengths λp and λs equal to about 1/1000.

[0233] Such U.V. trimming step is carried out in a way that issubstantially similar to what described in relation to FIG. 13.

[0234] As regards the production of the optical couplers of a couplingsection 20 of the type described with reference to FIG. 5 and thewriting of Bragg gratings 27, 27′ on the two upper 38 and lower 39 armsof said coupling section 20, such operations are carried out in a waysimilar to what described above in relation to FIGS. 10-13.

[0235] The pumping unit 10 according to the invention can be used in anyapplication in which it is necessary to combine two or more pump sourceswith a signal radiation at wavelength λs (or a WDM signal). For example,it can be used for obtaining a high pump power in an optical amplifier,or for obtaining a high Raman gain in an optical fibre.

[0236]FIG. 14a shows an optical amplifier, according to an embodiment ofthe invention, for amplifying a signal radiation at wavelength λs or aWDM signal comprising a plurality of signals at wavelengths λ1, λ2, . .. λN.

[0237] Such amplifier comprises a dielectric guiding active means 30 anda pumping unit 10 of the type described above with reference to FIGS.3a, 3 b, 4-8, having the fourth port 24 of the coupling section 20connected to the active means 30.

[0238] In the embodiment shown, the active means 30 is an active opticalfibre doped with a rare earth.

[0239] Among rare earths, erbium is the most frequently used componentbecause its fluorescence spectrum has a band comprised between 1420 and1650 nm, which corresponds to the third transmission window (centred atabout 1550 nm) of a telecommunication signal.

[0240] As an alternative, the active means 30 can comprise a substratumwith an active waveguide doped with a rare earth.

[0241] In this application, the first and the second pump radiation ofthe pumping unit 10 have wavelengths λp1 and λp2 corresponding to a peakof the absorption spectrum of the dopant substance of the active means30 whereas the signal radiation(s) has/have wavelength(s) correspondingto a metastable level of such dopant substance.

[0242] Pump radiations are thus capable of bringing the ions of thedopant substance to an excited energetic level. From such level, ionsfall spontaneously, in very short times, to a laser emission level ormetastable level, where they remain for a relatively longer time (namedmean lifetime of the metastable level).

[0243] When the signal radiation(s) having wavelengths corresponding tosuch metastable level pass through the active means 30 having a highnumber of excited ions on the metastable level, the excited ions fall toa lower level, thus causing a stimulated luminous emission having thesame wavelengths as the signal radiations.

[0244] In the embodiment of FIG. 14a the pumping unit 10 is of the typedescribed above with reference to FIG. 3b and the pump radiations travelin the active means 30 in a co-propagating direction with respect to thedirection of the signal radiation (co-propagating pumping).

[0245]FIG. 14b shows an alternative embodiment of the optical amplifier1 which is totally similar to that of FIG. 14a except in that thepumping unit 10 is of the type described above with reference to FIG. 3cand the pumping of the active means 30 occurs in a counter-propagatingdirection with respect to the direction of the signal radiation.

[0246] Should it be necessary to pump the optical amplifier 1 with morethan two pump sources, the pumping unit 10 comprises three or more pumpsources, and has a configuration similar to those of FIGS. 9a and 9 b.

[0247]FIG. 15 shows a telecommunication system 100 according to theinvention, comprising a transmitting station 120 for providing a signalradiation at a wavelength λs, a receiving station 140 for receiving saidsignal radiation, and an optical fibre transmission line 160 fortransmitting the signal radiation from the transmitting station 120 tothe receiving station 140.

[0248] According to a preferred embodiment, the telecommunication systemis a WDM system.

[0249] In this case, the transmitting station 120 is a conventional WDMequipment adapted to provide N signal radiations having wavelengths λ1,λ2 . . . λN which are different from one another, to wavelengthmultiplex them in a single WDM optical signal and to send such WDMoptical signal along the optical transmission line 160. Moreover, saidtransmitting station 120 also comprises an optical power amplifier(booster, not shown) for amplifying the WDM optical signal beforesending it along line 160 (or a certain number of boosters in parallel,for amplifying signal radiations comprised in different wavelengthbands).

[0250] Such wavelengths λ1, λ2 . . . λN are typically selected in aninterval of wavelengths comprised between 1520 nm and 1630 nm.

[0251] For example, the telecommunication system 100 can be a WDM systemwith 128 channels, spaced from one another by 50 GHz and divided intothree bands: 16 channels between 1529 and 1535 nm (first band); 48channels between 1541 and 1561 nm (second band) and 64 channels between1575 and 1602 nm (third band).

[0252] Said receiving station 140 comprises a conventional equipmentadapted to demultiplex said N optical signals and to send them tooptional successive processing stages. Moreover, said receiving station140 typically comprises also an optical preamplifier (not shown) adaptedto bring the WDM optical signal to a power level adapted to be receivedby the receiving equipment (or a certain number of optical preamplifiersin parallel for amplifying the signal radiations comprised in differentwavelength bands).

[0253] Line 160 comprises a plurality of optical amplification units180, each comprising an optical amplifier for amplifying a signal comingfrom an upstream line portion 160, wherein the signal has attenuatedduring its propagation along it, and sending it to a downstream lineportion 160.

[0254] Each unit 180 can also comprise a certain number of opticalamplifiers arranged in parallel for amplifying the signal radiationscomprised in different wavelength bands (for example, the first, thesecond and third band mentioned above).

[0255] For example, system 100 can be a submarine telecommunicationsystem, wherein line 160 comprises optical cables 160 ₁, 160 ₂, 160 ₃, .. . 160 _(n) which respectively connect the transmitting station 120 tothe first amplifier 180, such amplifier to the next one and the lastamplifier to the receiving station 140.

[0256] Each optical cable 160 ₁, 160 ₂, . . . 160 _(n) comprises atleast one optical fibre, and has a length which can vary from somedozens kilometres to some hundreds kilometres.

[0257] Such optical fibres preferably are single-mode at the Nwavelengths of transmission λ1, λ2 . . . λN and they are, for example,of the step index type.

[0258] At least one of the optical power amplifier of the transmittingstation 120, the preamplifier of the receiving station 140 and theoptical amplifiers of the amplification units 180 is an opticalamplifier according to the invention, of the type described above withreference to FIGS. 14a and 14 b.

[0259] The present invention has been described, by way of an example,with reference to optical pumping units. However, it can be equallyapplied to devices for mixing at least three radiations at differentwavelengths from one another in applications for treating WDM opticalsignals.

[0260] For example, the optical unit 10 of the invention can be used formixing three WDM signal bands (or three signal radiations) centred atabout 980 nm, 1530 nm and 1550 nm.

What is claimed is:
 1. Optical pumping unit comprising a first pumpsource adapted to emit a first pump radiation at wavelength λp1; asecond pump source adapted to emit a second pump radiation at wavelengthλp2, with the wavelength λp2 different from the wavelength λp1; and acommon coupling section comprising a first and a second port connectedto the first and second pump source for respectively receiving the firstand the second pump radiation; a third port for a signal radiation atwavelength λs; and a fourth port; wherein said coupling section isadapted to combine, in the fourth port, the signal radiation and thefirst and second pump radiation by means of a reversal of the directionof propagation of the first pump radiation from the first port to thefourth port.
 2. Pumping unit according to claim 1, wherein the couplingsection also comprises a first optical path which connects the first andthe second port; and a second optical path, in communication with thefirst optical path, which connects the third and the fourth port, and itis adapted to send to the fourth port the first pump radiation, whichpropagates along the first optical path from the first port to thesecond port, making it pass from the first optical path to the secondoptical path and reflecting it back towards the fourth port.
 3. Pumpingunit according to claim 2, wherein the coupling section is also adaptedto send to the fourth port the second pump radiation, which propagatesalong the first optical path from the second port towards the firstport, making it pass from the first optical path to the second opticalpath.
 4. Pumping unit according to claim 2, wherein the coupling sectionis also adapted to let the signal radiation propagate along the secondoptical path.
 5. Pumping unit according to claim 1, wherein saidcoupling section comprises an optical reflection element adapted toreflect the first pump radiation at wavelength λp1 towards the fourthport, and to let the second pump radiation at wavelength λp2 and thesignal radiation at wavelength λs pass.
 6. Optical pumping unitaccording to claim 5, wherein said optical reflection element is a Bragggrating.
 7. Optical pumping unit according to claim 2, wherein the firstoptical path comprises a waveguide.
 8. Optical pumping unit according toclaim 7, wherein the second optical path comprises a waveguide. 9.Optical pumping unit according to claim 8, wherein the first and thesecond optical path are coupled along a coupling area.
 10. Opticalpumping unit according to claim 9, wherein the coupling area is such asto let substantially all the power of the signal radiation at wavelengthλs propagate along the second optical path, and to let substantially allthe power of the first pump radiation at wavelength λp1 andsubstantially all the power of the second pump radiation at wavelengthλp2 pass from the first optical path to the second optical path. 11.Optical pumping unit according to claim 10, wherein the first and thesecond optical path form a WDM optical coupler of the 100% λp1,λp2/0% λstype, comprising two waveguides coupled with one another in saidcoupling area.
 12. Optical pumping unit according to claim 10, whereinsaid coupling section comprises an optical reflection element positionedin the coupling area of the first and the second optical path, adaptedto reflect the first pump radiation at wavelength λp1 towards the fourthport and to let the second pump radiation at λp2 and the signalradiation at wavelength λs pass.
 13. Optical pumping unit according toclaim 12 wherein said optical reflection element is a Bragg grating. 14.Optical pumping unit according to claim 12, wherein said opticalreflection element is positioned in a point of the coupling area atwhich about 50% of power of the first pump radiation passes from thefirst optical path to the second optical path.
 15. Optical pumping unitaccording to claim 9, wherein the first and the second optical path arealso coupled along a second coupling area.
 16. Optical pumping unitaccording to claim 15, wherein the first and the second optical pathcomprise an input coupler, an output coupler, an upper arm and a lowerarm, and wherein the input coupler has four ports of which two are thesecond and the third port of the coupling section, and two are incommunication with the upper arm and the lower arm, and the outputcoupler has four ports of which two are the first and the fourth port ofthe coupling section, and two are in communication with the upper armand the lower arm.
 17. Optical pumping unit according to claim 5,wherein the coupling section also comprises a second optical reflectionelement adapted to reflect the first pump radiation at wavelength λp1towards the fourth port, and to let the second pump radiation atwavelength λp2 and the signal radiation at wavelength λs pass. 18.Optical pumping unit according to claim 17, wherein the first and thesecond optical path comprise an input coupler, an ouput coupler, anupper arm and a lower arm, and wherein the input coupler has four portsof which two are the second and the third port of the coupling section,and two are in communication with the upper arm and the lower arm, andthe output coupler has four ports of which two are the first and thefourth port of the coupling section, and two are in communication withthe upper arm and the lower arm, and wherein the first opticalreflection element is positioned in said upper arm and the secondoptical reflection element is positioned in said lower arm.
 19. Opticalpumping unit according to claim 18, wherein the input coupler and theoutput coupler are two WDM optical couplers of the 50% λp1, λp2/0% λstype, each comprising two waveguides coupled with one another in saidfirst and said second coupling area.
 20. Optical amplifier foramplifying a signal radiation at wavelength λs comprising a dielectricguiding active means and a pumping unit according to claim 1, whereinthe fourth port of the coupling section is in communication with theactive means.
 21. Optical communication line comprising a transmissionoptical fibre length and a pumping unit according to claim 1, whereinthe fourth port of the coupling section is in communication with saidtransmission optical fibre length.
 22. Optical communication linecomprising a transmission optical fibre length and an optical amplifieraccording to claim 18 in communication with said transmission opticalfibre length.
 23. Optical communication system comprising a transmittingstation adapted to provide a signal radiation having wavelength λs; anoptical transmission line, optically connected to said transmittingstation, for transmitting said signal radiation; a receiving station,optically connected to said optical transmission line, for receivingsaid signal radiation; at least one pumping unit according to claim 1,in communication with said optical transmission line.
 24. Opticalcommunication system comprising a transmitting station adapted toprovide a signal radiation having wavelength λs; an optical transmissionline, optically connected to said transmitting station, for transmittingsaid signal radiation; a receiving station, optically connected to saidoptical transmission line, for receiving said signal radiation; at leastone optical amplifier according to claim 18 in communication with saidoptical transmission line.
 25. An optical coupling section, for couplinga signal radiation at wavelength λs, a first pump radiation atwavelength λp1 and a second pump radiation at wavelength λp2, comprisinga first and a second port for receiving respectively the first and thesecond pump radiation; a third port for the signal radiation; and afourth port, and being adapted to combine the signal radiation and thefirst and second pump radiation in the fourth port through a reversal ofthe direction of propagation of the first pump radiation from the firstport to the fourth port.
 26. Method for coupling a first radiation atwavelength λp1, a second radiation at wavelength λp2 and a thirdradiation at wavelength λs through a common coupling section having afirst and a second side that are opposed to one another, the first sidecomprising a first and a fourth port and the second side comprising asecond and a third port, said method comprising the steps of a)propagating the second radiation from the second port to the first port;b) deviating the path of the second radiation so as to send it to thefourth port; c) sending the third signal radiation from the third portto the fourth port, or vice versa, from the fourth port to the thirdport; d) propagating the first radiation from the first port to thesecond port; and e) reversing the direction of propagation of the firstradiation to send it to the fourth port.
 27. Method according to claim26, wherein the common coupling section also comprises a first opticalpath connecting the first and the second port, and a second opticalpath, in communication with the first optical path, connecting the thirdand the fourth port.
 28. Method according to claim 27, wherein step a)is carried out by sending the second radiation along the first opticalpath from the second port to the first port.
 29. Method according toclaim 28, wherein step b) is carried out by making the second radiationpass from the first optical path to the second optical path.
 30. Methodaccording to claim 25, wherein step c) is carried out by letting thethird radiation propagate along the second optical path.
 31. Methodaccording to claim 30, wherein step d) is carried out by sending thefirst radiation along the first optical path from the first port to thesecond port.
 32. Method according to claim 31, wherein step e) iscarried out by making the first radiation pass from the first opticalpath to the second optical path and back-reflecting it towards thefourth port.